1. Field of Invention
The present invention relates to casting metals. More particularly, the present invention relates to an apparatus and method for controlled optimized directional rapid solidification of metal castings to improve mechanical characteristics of the casted metal and the casting.
2. Background
The casting of molten metal into desired shapes has been known and practiced for centuries. Known molten metal casting processes involve pouring, or forcing, molten metal into a mold cavity. Through the loss of heat, the molten metal within the mold cavity solidifies creating a cast part in the shape of the mold cavity.
Molten metal casting employs a variety of techniques and a variety of apparatus, including, but not limited to permanent molds, sand casting, die casting, and investment casting.
Permanent molds, which may be formed of hardened steel, are well suited for casting detailed objects, having thin walls and intricate structures. One recognized drawback however is that permanent molds are expensive.
Sand casting is a process where a mold cavity (or depression) is formed in sand with a mold pattern having a shape similar to the desired casting. If the finished casting is to be hollow or define internal cavities or passages, such internal passages are formed with “cores” that are placed within the cavity/depression formed in the sand. Some sand casting methods use “green sand” which is a mixture of clay and moisture that together function as a binder for the sand aggregate. Other forms of sand binders, such as resins and chemicals may also be used for higher levels of hardness and improved accuracy. Unfortunately, sand binders are subject to Environmental Protection Agency (EPA) regulations and restrictions because the byproducts such sand binders generate during decomposition caused by heat are poisonous and/or hazardous. EPA regulations and restrictions have increased the cost of using resin and chemical binders in sand casting processes.
Die casting is a process similar to the permanent mold process where molten metal is forced, under high pressure, into a mold cavity which is formed by plural hardened steel dies which have been machined into a desired shape. The die casting process is somewhat similar to known injection molding processes.
Investment casting also known as the lost pattern process or lost foam process (hereinafter lost foam casting) is a method to cast metal items requiring smooth finished surfaces.
Lost foam casting is a variation of sand casting but is an improvement thereover because it provides better net shape capabilities as compared to green sand casting methods and eliminates parting lines and matching operations that increase costs. The lost foam process also provides better dimensional tolerances as compared to green sand castings because core shifts and core variabilities are eliminated. A direct benefit is a significant reduction in finish machining costs and infrastructure investment due to the high net shape of the casting, with less opportunities for errors in matching and assembly.
In the lost foam process, a pattern is made and thereafter sacrificed, when the molten metal is poured. A variety of pattern materials may be used, such as but not limited to wax, polystyrene foam, polyethylene foam, and other known pattern forming materials. For purposes of this disclosure, the patterns described herein are formed of foam, although it is to be understood that any known pattern making material may be used.
The lost foam process involves the formation of a foam pattern by injection of foam into a cavity defined by a die so that the foam completely fills the die cavity and conforms to the interior shape of the die cavity. Plural individually formed foam patterns may be combined to form a single complex pattern. The pattern is thereafter dipped into a medium that forms a coating covering the pattern. The coating is dried, resulting in a hardened exterior surface encasing the pattern. Thereafter, the coated pattern is placed in a mold flask, also known as a mold container. Backing media, such as sand, is packed around the coated pattern to provide support for the coated pattern when molten metal is added. Typically the backing media is packed around the coated pattern using a known mechanical vibration means such as a vibration table upon which the mold flask containing the coated pattern and backing media is placed. Because the “loaded” mold flask is a massive and heavy assembly it typically requires significant industrial infrastructure to maneuver and process. Sand which is the most commonly used backing media is heavy weighing on average between 100-120 lbs. per cubic foot. Further, sand is not eclectically conductive and has low thermal conductivity and is not conducive so it does not provide rapid heat dissipation.
A crucible or similar vessel containing molten metal is used to pour molten metal into a runner that communicates with the pattern. As the molten metal contacts the foam pattern, the foam decomposes and is vaporized. The molten metal replaces the foam pattern within the hardened coating which maintains the desired shape of the casting and the desired surface characteristics. The backing media surrounding the coated pattern provides stability while the metal cools and solidifies.
The mold flask is thereafter set aside to allow the molten metal to cool and solidify. This process is known as “freezing”. Once freezing is complete, the cast part may be removed from the mold flask. As mentioned previously, sand has low thermal conductivity and, as a result, each casting takes a significant amount of time freeze. (e.g. multiple hours). The hardened coating is removed from the cast part by a variety of processes known to those skilled in the art and the backing media (the sand) is typically reclaimed and reused. The cast part may then be tempered by a variety of means and methods and thereafter finish machining may occur.
Unfortunately, the lost foam process has known drawbacks. The vaporization of the foam pattern generates fumes that are noxious and/or poisonous. The decomposing foam pattern cools the molten metal which may cause defects within the cast part and may also release hydrogen gas, which may be captured within the molten metal as it freezes, causing defects in the cast part. The absence of uniform density of the foam may prevent smooth and predictable filling of the mold cavity allowing the molten metal to advance more rapidly in one section of the mold cavity and then “fold back” as other sections of the mold cavity are filled, thereby enfolding defects within the casting.
Another drawback to the lost foam process is associated with the slow cooling of the cast metal. As described previously, after the molten metal is poured into the mold cavity, the filled mold and surrounding mold flask are set aside until sufficient heat has dissipated from the metal so that the metal has solidified, whereupon the casting within the mold cavity may be removed from the mold flask. This period of time may be lengthy, (multiple hours) and the slow cooling of the molten metal is known to cause low quality castings which do not provide desirable and sought-after mechanical properties, and may include inferior granular micro-structures within the casted piece. Movement of the massive, heavy and hot mold flask requires specialized lifting equipment and the length of time for the freeze to occur necessitates a long production cycle because the mold flasks cannot be utilized again until the freeze has occurred and the casting and mold media has been removed from the flask.
Another drawback to known lost foam casting, and slow cooling, is that the casting requires subsequent solution heat treatment, which increases costs and increases production time.
An event further drawback to known lost foam casting process is that the solidification of the molten metal is uncontrolled. (i.e. is not directional).
A variety of means, methods and apparatus have been attempted to improve known lost foam casting processes but the drawbacks have remained. One example of an improvement to lost foam casting uses sand aggregates and a water soluble binder and is known as the “ablation casting process” disclosed in U.S. patent application Ser. No. 11/505,019 (Patent Publication No.: US 2008/0041499A1). In the ablation process, rapid solidification of the cast piece is achieved by cooling the sand mold with water similar to a “carwash” that washes away the backing sand and water soluble binders (ablation) to speed the cooling.
Although the ablation process is an improvement over prior methods and apparatus and is more controlled than “submerging” a mold flask containing a casting into a tub of coolant, there continue to be various recognized problems, difficulties and drawbacks with lost foam casting, including, but not limited to: (1) controlling the mold cavity filling; (2) weak mold patterns; (3) hydrogen porosity control; and (4) fold defects. For example and without limitation, the ablation process does not provide a means for creating controlled multi-directional intersecting freeze-fronts in a casting and the ablation process is extremely “messy” and requires installation and maintenance of the “car wash” apparatus into which the hot flasks are “fed”.
As noted previously, the mold backing media commonly used in lost foam casting is sand. Most commonly, the sand used is silicon or zircon, both of which have low thermal conductivity such that heat is not conducted away from the molten metal causing a slow solidification rate. This is a significant drawback because it is well-known that rapid cooling of the molten metal is desirable because of improved mechanical properties of the casting, as well as the number of operation steps that may be eliminated with rapid cooling. Moreover, rapid cooling causes retention of a greater portion of the alloying elements in solution and may also allow elimination of subsequent heat treatments, which saves time and expense.
Various methods and techniques to overcome and improve the mechanical properties of lost foam cast items have been attempted, but there remains a significant need for a “lost foam” casting process that provides for rapid directional cooling and rapid directional solidification of the cast piece while simultaneously providing desired mechanical properties of the casted item that result from rapid cooling and rapid solidification.
As noted previously, known “lost foam” casting processes produce cast items having low to average mechanical properties because of the slow cooling and slow solidification rate and as a result, items formed through known “lost foam” casting processes, are typically not capable of being used in applications requiring high-strength, such as aerospace applications and military applications.
What is needed is an apparatus and method that provides all the advantages of lost foam casting with controllable optimized thermal management that provides optimized rapid cooling and optimized controlled directional solidification, including directional solidification from multiple directions.
My apparatus and method overcomes various of the aforementioned drawbacks to the lost foam process by providing improved mold flasks that are used in conjunction with a new mold media and an improved cooling method that provides for controlled optimized directional rapid cooling and optimized solidification of molten metal castings.
My mold backing media is synthetic carbon graphite. My carbon graphite mold media is light weight, weighing on average about 3-15 lbs. per cubic foot. My carbon graphite mold media is highly thermally conductive with a conductivity of up to 1,700 W/m·K (watts per meter Kelvin) as compared to Copper which has a thermal conductivity of approximately 400 W/m·K. Further, my carbon graphite mold media is highly electrically conductive nearly matching the electrical conductivity of copper. My carbon graphite mold media is produced by high-temperature treatment of amorphous carbon materials. The primary feedstock for making synthetic graphite is calcined petroleum coke and coal tar pitch, both of which are highly graphitizable forms of carbon. The manufacturing process consists of mixing, molding, and baking operations followed by heat-treatment to temperatures between approximately 2500C and 3000C. The heat drives the solid/solid, amorphous carbon-to-graphite phase transformation. The morphology of synthetic graphite varies from “flaky” in fine powders to irregular round grains in coarser products which is caused by high temperature vaporization of volatile impurities, which include most metal oxides, sulfur, nitrogen, hydrogen, and all organic components that were part of the original petroleum or coal tar pitch. The available particle size range is generally from approximately small 2-micrometer powders to 3 cm pieces, although larger particles may also be obtained. The “near spherical” shape is a preferred embodiment because of the need for “spaces” and “gaps” between the individual particles when supporting a mold in a mold flask. In an alternate embodiment, the thermally and electrically conductive mold backing media may be mixed with silica sand, or other larger mostly round in shape mold material medias in the interest of economics. Copper coated synthetic graphite or other materials such as copper coated “Iron Buck-shot” is another alternate mold material media because of its conductivity. The exterior shape of the mold media particulates facilitates deployment and passage of coolants.
My invention provides a mold flask that operates with a coolant system for controlled optimized directional rapid freezing of molten metal.
My invention provides mold media that is electrically conductive and thermally conductive for heating the mold prior to pouring molten metal into the mold, and also to quickly dissipate heat from the molten metal to provide rapid cooling and a controlled solidification freeze front.
My invention provides mold media that is recyclable, does not use binders and does not generate polluting or other dangerous byproducts.
My invention provides mold media that is aggregate, defining generally spherical shaped particles that allow coolant to permeate through the media to draw heat away from the molten metal cast.
My invention provides for controlled rapid directional cooling of a molten metal casting to enhance the mechanical characteristics and optimizes the solidification cooling rate by the temperature gradient affecting the alloying elements within the molten metal within the mold cavity.
My invention provides an apparatus and method that is operable with liquid coolants and gaseous coolants.
Some or all of the problems, difficulties and drawbacks identified above and other problems, difficulties, and drawbacks may be helped or solved by the inventions shown and described herein. My invention may also be used to address other problems, difficulties, and drawbacks not set out above or which are only understood or appreciated at a later time. The future may also bring to light currently unknown or unrecognized benefits which may be appreciated, or more fully appreciated, in the future associated with the novel inventions shown and described herein.